A(1) Field of the Invention
The invention relates to a system comprising one or more transmitters and receivers for the transmission of messages. The system is particularly intended as a remote control system for setting a variety of functions of one or more apparatus each comprising such a receiver for this purpose.
Such a system will hereinafter be referred to as RC system, a transmitter used therein will be referred to as RC transmitter and a receiver will be referred to as RC receiver.
It is to be noted that an apparatus may herein be understood to mean a TV receiver, a video recorder, a teletext decoder, an audio amplifier, an audio tuner, a luminaire, a door, etc.
A(2) Description of the Prior Art
In a remote control system the RC transmitter is provided with a message generator which upon request delivers a message in the form of a series of pulses. For the transmittal of such a series of pulses an infrared modulation circuit is provided which comprises an infrared LED whose emitted infrared light is modulated by these pulses.
More particularly, each message comprises an address word indicating for which apparatus the message is intended and a command word indicating which function of the selected apparatus must be set and to which value it must be set. An address word and a command word combined are unique for the message to be transmitted.
The RC receiver is provided with an infrared demodulation circuit comprising an infrared photodiode. It converts received infrared light into an electric signal which is a rough version of the transmitted message. This signal is applied to a signal processing circuit for generating a stylized version of the message, that is to say, a version whose form corresponds to the original message, hence with clearly defined pulses.
Embodiments of the above-mentioned RC transmitters and RC receivers have been extensively described in References 1, 2 and 3 defined more particularly, hereinafter. As indicated in these References, the command word and address word in each message is usually preceded by one or more auxiliary pulses such as, for example, a starting pulse. In Reference 3 it is also proposed to transmit a wake-up pulse preceding the starting pulse. Unfortunately, its purpose is not revealed in this publication.
The RC transmitters and RC receivers described in the forementioned References are eminently suitable in practice, at least if they are used in a so-called one way RC system. This is an RC system with a central RC transmitter and, remote therefrom, a plurality of local RC receivers. The central RC transmitter forms part of a hand-held unit and is battery-fed, whilst each local RC receiver forms part of an apparatus to be operated and is usually fed from the mains.
In recent years apparatus to be operated have increasingly been provided with a local RC transmitter so that such an apparatus can also transmit messages to either a local RC receiver or to the hand held unit which is then provided with a central RC receiver. In this way, in response to a message transmitted by the central RC transmitter, a local RC transmitter can transmit a reply message intended for the central RC receiver which processes this reply message and notifies the user, for example, by means of a piece of information on a display.
Such a system is referred to as a two-way remote control system. In addition to the above-described situation in which the local RC transmitter transmits a message in response to a message received from the central RC transmitter the aim is to provide apparatus with a facility for transmitting messages autonomously. Since it is then not known in advance when an apparatus transmits a message, all RC receivers will have to be continuously in an operative state. For the currently used RC receivers this implies that approximately 50 mAh is consumed every 24 hours. This is no drawback for those RC receivers receiving their energy from the mains, but it is admissably high for RC receivers receiving their energy from a battery, such as the central RC receiver in the hand held unit. In fact, the present-day batteries have an energy content of approximately 480 mAh so that in a two-way RC system the batteries in the hand held unit will be exhausted after approximately one week. This is in contrast to a one-way RC system in which the batteries in the hand-held unit may last as long as four years.